For detecting the level of liquids or bulk materials in containers, contactless measuring systems are increasingly used. As the measurement radiation, microwaves, ultrasound waves, electromagnetic pulses, light pulses, or—in particularly critical applications—radioactive rays are used.
In many areas of use, such as petroleum chemistry, chemistry, and the foods industry, high-precision measurements of the level of liquids or bulk materials in containers (tanks, silos, etc.) are needed. Increasingly, TDR sensors are therefore used, in which brief electromagnetic high-frequency pulses or continuous microwaves are input into a conductive elongated element, such as a rod probe or cable probe, and are introduced through the conductive element into the container in which the product is stored. TDR is the abbreviation for time domain reflectometry.
In terms of physics, in this measuring method the effect is utilized that at the boundary face of two different media, such as air and oil, or air and water, because of the abrupt change (discontinuity) in the dielectric constants of the two media, some of the guided electromagnetic pulses or the guided microwaves are reflected and returned to a receiving device via the conductive element. The reflected portion of the electromagnetic pulses or microwaves is greater, the more different the dielectric constants of the two media are. The distance from the boundary face can be determined from the propagation time of the pulses or waves. If the empty distance of the container is known, then the level of product in the container can be calculated afterward.
Sensors with guided high-frequency signals (pulses or waves) are distinguished over sensors that freely broadcast high-frequency pulses or waves (free-field microwave systems or FMR, also called “genuine radar systems”) by substantially lower attenuation. The reason for this is that the power flow is effected quite intentionally along the rod or cable probe or conductive element. Moreover, sensors with guided high-frequency signals in the near range have a higher measurement quality than freely broadcasting sensors.
Another advantage of sensors with guided high-frequency signals is the high degree of safety and reliability of the level measurement performed. This is because the measurement with guided transmission signals is more independent of the properties of the product, the container construction (such as its materials and geometry), or other operating conditions (such as dust and deposits).
From U.S. Pat. No. 5,233,352, a level measuring device has been disclosed in which two pulse generators generate two binary pulse trains. The second pulse train is delayed compared to the first pulse train, and the time lag is designed to be variable and is dimensioned such that the time lag as a consequence of the propagation time of the first pulse train is equal to the set time lag between the two pulse trains. The correct time lag is ascertained by means of autocorrelation of the two pulse trains.
One disadvantage of this disclosed embodied is considered to be that for operating the known measuring device, high-frequency pulse trains have to be used. Only in that way can high enough measurement accuracy be achieved.